THE opening of “The September Issue,” R.J. Cutler’s documentary about Vogue, is possibly the cruelest couple of moments in fashion.

It features Anna Wintour, editor-in-chief of the magazine and sunglasses-wearing poster child of Fashion with a capital F, a woman facing 60 in a business that fetishizes youth, in extreme, line-defining close-up, making defensive statements about why people are intimidated by fashion. And she does so in the most intimidating way.

Those first few seconds are akin to sitting in on somebody’s therapy session, so naked is Wintour’s frustration suffering fools, and it goes a long way in explaining why she doesn’t do many interviews.

That the most powerful and protected woman in fashion does so now — in this film, on “60 Minutes” earlier this spring, on “Letterman” next week — is a mystery. Except that after 20 years, with fashion in economic crisis, management consultants turning Condé Nast inside out, vulture critics circling and speculating about her own exit strategy, she must be thinking in terms of legacy.

For the documentary, which opens Aug. 28, the magazine — which is to say Wintour — allowed Cutler an extraordinary level of access for the closing of the most important issue of the year in 2007, which at 840 pages was the magazine’s biggest ever. Cutler followed Wintour and her team from the shows in February — when fall clothes are shown on world stages — from planning with editors and photographers, fashion shoots, meetings with retailers and designers, to closing in July, interspersed with interviews at her homes in Greenwich Village and Long Island.

This peek inside the star chamber is juicy viewing on a number of levels. It’s a psychological portrait of Anna, powerful female executive, mother, daughter, perfectionist. It’s a front-row seat at how the albeit-impeccably-turned-out-but-sausage-nonetheless gets made at Vogue.

And perhaps most interestingly, it’s a snapshot of Paris before the Revolution, before the bottom fell out of the Park Avenue parquet, the world Wintour courted and documented so finely in the pages of her magazine.

Cut to the $2 million-a-year editor sipping her Starbucks in the back of a chaffeur-driven limousine that is her daily commute as the examples of a soon-to-be bygone era unfold.

As the pressure of producing a blockbuster issue mounts, Wintour jettisons $50,000 worth of photos from a shoot. One minute a designer dress is on a rack in the halls of Vogue, the next it is on her back. Heraldic assistants sounding the alarm of her arrival contrast nicely with viewers’ knowledge that, in real life, Condé Nast receptionists were all recently laid off.

Even if you don’t give a fig for fashion, it’s rare that you get to see Nero tuning up his fiddle as Rome is about to spontaneously combust.

And to be sure, she is a dictator — that we already know. What the viewer can now decide is if she is, in fact, a benevolent dictator.

Neiman Marcus must surely think so, when it’s revealed that, at the behest of the retailer, Vogue editors “convinced” Miuccia Prada to change a fabric. That Oscar de la Renta agrees is less certain when she witheringly tells the king of American couture that one of his designs is — shudder — “less exciting.”

Indeed, her very humanity is always at issue. At an Yves Saint Laurent showroom preview, she can barely contain her contempt for a colorless collection. When she publicly dresses down an editor for the “sameness” of her ideas, the staffer looks like she’s been kicked in the stomach.

Even Tom Florio — who, although he’s the publisher, clearly sits a little lower in the pantheon — admits he “has to be warm enough for both of them,” looking like his life is flashing before his eyes even as he says it. Let’s just say she’s not a people- person.

The question the film begs — but cannot answer — is what will Wintour’s future look like in this new economic and cultural reality? This year Vogue has been trumpeting a number of budget-minded stories in response to the economy on its covers — “Steal of the Month,” “Fashion’s Quick Fix for $19,” “How to Splurge & Where to Save” — and you don’t have to be Anna Wintour to know those teasers are tacky. This September’s issue will be considerably thinner, as advertising pages have declined 37 percent from a year ago. And outtakes from her “60 Minutes” interview carried a whiff of Marie Antoinette’s cake recommendation with her remarks about people in Minnesota shaped like “little houses.” As management consultants crawl all over Wintour’s office turning over every garment bag and expense report, will the industry that is her power base, as it shrinks and resets, still agree that the empress is dressed to the nines?

There are approximately two times in the hour and half of “The September Issue” in which warmth crosses Wintour’s face: one when she is looking at Bee Shaffer, her beautiful show pony of a daughter, her constant accessory at fashion shows and public appearances — who gives a hilarious interview dissing her mother’s business. The other is when she talks about Grace Coddington, Vogue’s 68-year-old, flame-haired creative director, the Mary Stuart to Wintour’s Elizabeth I, and the only member of court from which she’ll brook dissent. (Their face-off over a shoot is one of the film’s choicest moments.)

What drives this fashion sphinx of a woman? As uncomfortable as it is, she ends up revealing that, too. Wintour awkwardly talks about her brothers and sister pursuing more altruistic endeavors and what they think of her high-flying career, of her cold British newspaper editor of a father who decided for her what she would do when she grew up. In the film she calls him “inscrutable” and has expressed her admiration for him.

In the final analysis, the all-powerful editor of Vogue may be just another girl with Daddy issues.